Piezoelectric thick films for microelectromechanical systems application

Abstract

This thesis concerns the processing and characterization of thick PZT sol-gel films for potential applications in MEMS devices. The deposition method was spin-coating. The aim was to reduce the number of coatings in the film processing by increasing the thickness of a single coating, with the restriction that the processed films must be crackfree and dense. Only by retaining the thick film dense, pore-free and crack-free one can obtain the piezoelectric properties in thick films that make the PZT thin sol-gel films attractive for the MEMS applications. Three PZT compositions, PZT 40/60, PZT 60/40 and PZT 52/48 were investigated. Each one of these PZT compositions has different crystallographic structure and thus differences in the piezoelectric properties were expected. The processing of thickness-increased sol-gel films was investigated. A combination of analysis techniques was employed. The stress development was monitored via ex-situ wafer deflection measurement after various fabrication steps. The ongoing processes in the sol-gel film were identified and correlated to certain temperature ranges and to the stress that is induced with each process in the film. It was found that crack-free films could be fabricated if a stress-controlled heating profile was applied. The PZT films were deposited on platinised silicon substrate and it was found that stress-related recrystallization was taking place in the platinum electrode which affected the total stress. After the platinum recrystallization the stress state in the bottom electrode and in the substrate was stable. Films up to 5 μm thick were obtained by repeated deposition of 200 nm thick single layers. The maximum thickness of a single coating was increased to 500 nm and a 2.5 μm film was fabricated by only 5 repeated coatings. The crystallographic orientation of all three employed PZT compositions was studied systematically on Pt/Si substrate at different thicknesses. Also, individual PZT films were deposited onto platinised sapphire substrates, or on LNO/Si substrate. It was found that the orientation of the films changes gradually with each coating. The residual stresses in all three PZT compositions were studied. A stress model for composite structures was applied for the first time in PZT films stress analysis. The results have shown that the residual stress at the room temperature is due to thermal expansion mismatch between the individual layers. Furthermore, a large orientation dependent stress variation was found in PZT 52/48 films that could be only explained if anisotropic thermal expansion in PZT were considered. The lattice parameters of all PZT compositions were determined and were in good agreement with the residual stress results. Thus, using the stress model it was possible to understand the origin of stress in PZT films. Finally, the electrical properties of the PZT films were determined. It was found that the piezoelectric, dielectric and ferroelectric properties of PZT films vary with PZT composition, film thickness and depend on the substrate type. Based on the finding it was proposed that there must be an interfacial layer that is responsible for domain wall pinning and thus reduced PZT properties in films below 5 μm thickness. In thick PZT 40/60 films enhanced piezoelectric properties were found making these PZT compositions very promising candidates for MEMS application.